Conventionally, a vehicle brake device using a brake-by-wire technology is known which uses a hydraulic cylinder actuated by an electric actuator such as an electric motor to produce a brake pressure. It is known that the braking force tends to be reduced toward a latter part of a braking action when the brake pedal is depressed by a constant stroke owing to the mechanical property of the brake device, and this may prevent the vehicle from slowing down at a deceleration intended by the vehicle operator. Therefore, the vehicle operator has to depress the brake pedal further in order to achieve a desired deceleration, and this may complicate the braking operation for the vehicle operator.
In the case of the brake-by-wire system which uses an electric actuator configured to be operated so as to produce a brake fluid pressure corresponding to the depression stroke of the brake pedal (braking operation amount), in order to overcome such a problem, the electric actuator may be controlled such that the braking force or the deceleration is increased as the vehicle speed decreases during the course of the braking action so that a decrease in the braking force or a decline in the deceleration owing to the reduction in the frictional coefficient may be avoided (or the so-called buildup effect may be enhanced). See Patent Document 1, for instance.
According to the technology disclosed in Patent Document 1, the ratio of the target braking force (brake fluid pressure) or the target deceleration to the depression stroke of the brake pedal (braking operation amount) is varied depending on the vehicle speed. In particular, the ratio of the target braking force (target deceleration) to the depression stroke of the brake pedal is increased with a decrease in the vehicle speed so that the target braking force (target deceleration) increases with a decrease in the vehicle speed even though the depression stroke of the brake pedal is constant. It is said that a good braking action can be thereby achieved in a late phase of the braking action.